It is hard to know from a single static low-res picture, but my guess is that something caused a short across the high voltage lines. That could be a falling tree limb, a squirrel making an unfortunate hop, a bird with outstretched wings in the wrong place, some conductive debris blown across the wires by the wind, etc. It happens. I once saw it happen when a damp football got thrown such that it bridged enough of the gap between two wires.
Most of the time, this causes excessive current, which trips a timed breaker to shut off for a few seconds. That's usually enough time for the debris, charred squirrel, or whatever to fall away. Everything is fine again when the breaker turns back on.
However, sometimes the breaker doesn't trip. I'm not sure why exactly. Since ionized air conducts, the arc can continue after that. 60 Hz voltage does go to zero 120 times per second, but the ionization of the air, and therefore its conductivity can persist long enough to catch the next half-cycle.
In the case of a continuing arc, the arc usually travels to one end of the span. It will be drawn to where the wires are closer together, but it sometimes moves in seemingly strange ways. When the arc gets to a power pole, it can go out, or turn into a bigger problem. In the case of the football I mentioned earlier, the arc moved to a transformer. When it got there, there was a big bang, and power went out for the neighborhood. We ran out of there real fast, hoping nobody would notice one football was missing.
did you look at the linked video? It looked pretty wild, moving down the line!
No I hadn't, since any information pertinent to the question should be directly in the question.
However, I did look at the video just now. Note that the ground is wet. It recently rained, and there are clouds in the sky. Most likely the humidity was high, so the breakdown voltage of air was lower than usual.
Apparently there was something at the left end of what we can see in the video that partially bridged the air gap between two high voltage conductors. This was enough to cause an arc, but not a short sufficient to trip the nearest breaker.
The rest is pretty much as I described above. The arc had a preference to traveling to the right for some reason. In this case, it was able to continue across several power poles.